Many applications of power integrated circuits require high side driver configurations such as the one illustrated in prior art circuit 10 of FIG. 1. A power MOSFET 16 has a source coupled to load 18 and a drain coupled to Vcc through inductance 12 and resistance 14 due to cabling. Output power transistors such as transistor 16 in FIG. 1 are often used in automotive applications where a power supply, Vcc, may be connected with an appreciable amount of cabling causing a cable inductance 12 and cable resistance 14. Cable inductance 12 often causes problems when, due to a short circuit across load 18, transistor 16 must be turned off quickly. A fast turn-off of power transistor 16 gives rise to a large inductive flyback since current cannot change instantaneously through an inductor which is well understood by those skilled in the art.
FIG. 1 illustrates a common prior art solution to this problem. A short circuit detect 28 monitors load 18. In the event that load 18 becomes short circuited, as, for example, an automechanic in repairing a vehicle may short load 18 with a screwdriver, short circuit detect 28 manipulates a switch 20 to discharge the voltage on the gate of transistor 16 thus turning transistor 16 off. This "fast turn-off" triggers an inductive flyback at the drain terminal of power MOSFET 16, therefore the drain terminal voltage will decrease to a large negative value very quickly when power MOSFET 16 turns off. When the inductive flyback voltage reaches the breakdown voltage of power MOSFET 16, MOSFET 16 begins to conduct in the reverse biased avalanche mode and will dissipate all the energy that was stored in inductance 12 during the conduction cycle.
The circuit of FIG. 1 has a serious drawback; namely a reliability issue. When power MOSFET 16 turns off and experiences inductive flyback, it breaks down. When breakdown occurs the transistor junction breaks down nonuniformly causing "hot spots" to appear at junctions in MOSFET 16 where current has concentrated in localized areas. High current concentrations in localized areas lead to transistor damage and reliability problems.
As power integrated circuits switch larger currents and dissipate larger amounts of energy, better methods must be developed to provide switching reliability at a competitive cost.
It, accordingly, is an object of this invention to provide a circuit and method of dissipating the stored energy in inductances during switching that will protect the output device, conserve die area, and provide circuit reliability. Other objects and advantages of the invention will be apparent to those of ordinary skill in the art having reference to the following specification and drawings.